Java fern is almost impossible to kill in my experience and it grows like a weed, just don't plant it's root system, anchor it loosely to a nice piece of wood. Also, with decent inexpensive lighting java fern requires little expense in terms of care and if you know anyone with java fern in their tank they'll probably give you some for free... It grows like crazy and sends off little babies from the leaf tips regularly. Anubis is also easy to grow, but really slow growing and if you anchor it to a rock or a piece of driftwood it's pretty sturdy in the tank.